1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device including one or more guard rings for ensuring a desired breakdown voltage characteristic.
2. Description of the Background Art
Generally, a semiconductor device keeping a predetermined voltage by a pn junction or Schottky junction includes one or more guard rings for ensuring a desired breakdown voltage characteristic. Referring to FIGS. 20 and 21, a common semiconductor device that keeps a predetermined voltage by a pn junction and includes a plurality of guard rings will be described.
This semiconductor device includes an n-type semiconductor substrate 101 and a p-type semiconductor region 102 extending inward to a predetermined depth from one main surface of n-type semiconductor substrate 101. Between n-type semiconductor substrate 101 and p-type semiconductor region 102, an annular pn junction region (main junction region) 108 is formed. An anode electrode 113 is disposed in contact with a surface of p-type semiconductor region 102. On another main surface of n-type semiconductor substrate 101, a cathode electrode 114 is disposed in contact with n-type semiconductor substrate 101.
In a peripheral region in the semiconductor device, a plurality of p-type guard rings 103, 104, 105, 106 (hereinafter these guard rings will also be collectively referred to as p-type guard ring region 107) are arranged annularly. P-type guard ring region 107 is arranged such that the constituent guard rings are spaced at predetermined intervals and surround pn junction region 108.
The one main surface of n-type semiconductor substrate 101 is covered with an insulating film 109. A conductive film 111 is electrically connected with p-type guard ring region 107 via a contact hole 110 running through insulating film 109.
When a voltage is applied to such a semiconductor device, p-type guard ring region 107 serves to allow a surface potential of insulating film 109 near p-type guard ring region 107 to be identical to a potential of p-type guard ring region 107. In other words, a depletion layer uniformly extends rightward (FIG. 21), so that a potential difference between an upper portion and a lower portion of insulating film 109 (FIG. 21) can be decreased. Accordingly, an electric field of the pn junction is alleviated and a desired breakdown voltage characteristic of the semiconductor device can be ensured.
Here, if the voltage applied to the semiconductor device is smaller, the extent of the depletion layer is also smaller. In this case, the number of guard rings required for ensuring a desired breakdown voltage may also be smaller. If, however, a higher voltage of 600 V or more is applied, the depletion layer has to be extended further in order to ensure a desired breakdown voltage characteristic. Accordingly, the number of guard rings constituting the guard ring region has to be increased. The increased number of guard rings results in a larger area of the semiconductor device. If, for example, a breakdown voltage characteristic of 1200 V of the semiconductor device using silicon is to be ensured, five or more guard rings are necessary and the width of the guard ring region itself is 200 μm or more.
A technique has been proposed for the purpose of reducing the surface area of the semiconductor device by, for example, making smaller the distance between guard rings constituting the guard ring region. If the distance between the guard rings is decreased, the distance between conductive films electrically connected with respective guard rings is also decreased. Japanese Patent Laying-Open No. 2003-078138 discloses a relevant technique for reducing the surface area of a semiconductor device.